The upcoming next generation “5G” Millimeter Wave (mmWave) small cell is expected to coexist with microwave (e.g., E-UTRAN) macro cells for a long time. The macro-assisted mmWave cellular systems exploit the fact that mmWave small cells and microwave macro cells may compensate each other very well in coverage area, link capacity, spectrum availability, and service robustness.
The mmWave bands above 10 GHz offer abundant spectrum, possibly at license free or at least efficiently shared, where bands may span contiguously for hundreds of megahertz or even gigahertz. Comparatively, microwave bands below 6 GHz are known for suffering from shortage, known as “bandwidth crunch”, and are fragmented bands of tens of megahertz with expensive licensing.
Due to the high carrier frequency, an mmWave system enjoys the natural compactness of RF system design of potentially tens or hundreds of antennas in a very small area, but it also has the physical barriers of poor penetration, small channel coherence time, big propagation loss due to atmosphere gaseous losses and precipitation attenuation. That is why mmWave systems usually need highly directional beamforming techniques to meet the tight link budget even for a small cell coverage. Comparatively, microwave systems have much smaller propagation path loss, and hence wider coverage, but greater multipath dissipation and scattering that may translate to severe inter-cell interference.
Given different channel characteristics, at the radio access system level, mmWave systems may promise gigabit-rate links within limited (small cell) coverage that are yet coupled with challenging beam-tracking and intermittent links in low-to-medium mobility in particular. On the other hand, the legacy microwave systems offer proven record of robust wide-area coverage, e.g., macrocell services even for high mobility users at a service rate of up to hundreds of megabits.
From both network and radio access's perspectives, mmWave is currently considered a very promising choice for in-door or out-of-door “5G” cellular small cells, which may compensate microwave macro-cell in shortage of spectrum or in need for economical high-speed data services. In particular, the small cells offer downlink (DL) throughput boosting or coverage extension for an umbrella macro-cell at its edge. On the other hand, macro-cell coverage makes up mmWave's directional coverage limitation and bursty link disruption by offering reliable omni-directional overlay services for time-critical or mission-critical control signaling, or offering more robust and seamless services for low-rate high-mobility voice users. Together they constitute a layered communication infrastructure that promise reliability, wide coverage, economical yet diversified mobile QoS services.
The gradual deployment of mmWave small cell systems, just as many other previous wireless systems, may initially be standalone Greenfield or macro-assisted hotspots, then clusters of contiguous small cells overlaid on top of existing macro cells, and eventually large scales of dense deployment of mmWave small cells to host many stationary or mobile users, e.g., in stadiums or urban areas under a central controller.
Overall, macro-assisted mmWave small cells demand a scalable multi-RAT integration architecture that affect both UE and networks. Control and data plane shall be separated because they may not always go through the same radio access. Design of control and user plane architecture shall be scalable and consider 5G mmWave deployment scenarios. Clustered or Dense deployment of mmWave small cells under the coverage of a macrocell with the following characteristics are a design focus: UEs are of dual active RFs in mmWave band and microwave bands; no ideal backhaul link between small cell BS (SBS) and macrocell BS (MBS); dense mmWave small cells and UEs under an umbrella microwave macrocell; mmWave smallcell is similar to LTE at upper layers but like a new RAT otherwise at lower layers; and mmWave links offer Gbps rate but with intermittent connectivity.
The existing LTE HetNet Dual Connectivity (DuCo) architecture is not fine-tuned for mmWave small cells that have new radio characteristics and face new 5G requirements as well. The LTE DuCo architecture is designed only for some less dense deployed, relatively low-rate microwave smallcell scenarios, and not optimized for stationary or dense scenarios with Gbps mmWave small cells. A new user plane architecture to integrate mmWave small cells and microwave macro cells effectively is sought.